A physical model for the transport and sorting of fine-grained sediment by turbidity currents

Turbidite muds in cores from the outer Scotian continental margin, off eastern Canada, contain abundant thin silt laminae. Graded laminated units are recognized in parts of this sequence. These represent single depositional events, and show a regular decrease in modal grain size and thickness of the silt laminae through the unit. A similar fining trend is shown by both silt and mud layers over hundreds of kilometres downslope. Textural analysis of individual laminae allows the construction of a dynamically consistent physical model for transport and sorting in muddy turbidity currents. Hydraulic sorting aggregates finer material to the top and tail regions of a large turbidity flow which then overspills its channel banks. Downslope lateral sorting occurs with preferential deposition of coarser silt grains and larger mud flocs. Depositional sorting by increased shear in the boundary layer separates clay flocs from silt grains and results in a regular mud/silt lamination. Estimates can be made of the physical parameters of the turbidity flows involved. They are a minimum of several hundreds of metres thick, have low concentrations (of the order of 10^?3 or 2500 mg 1^?1), and move downslope at velocities of 10-20 cm s^?1. A 5 mm thick, coarse silt lamina takes about 10 h to deposit, and the subsequent mud layer ‘blankets’ very rapidly over this. A complete unit is deposited in 2-6 days which is the time it takes for the turbidity flow to pass a particular point. These thick, dilute, low-velocity flows are significantly different from the ‘classical’ turbidity current. However, there is mounting evidence in support of the new concept from laboratory observations and direct field measurements.     

Chemistry and pollution of natural waters in western Kenya

Selected water analyses from the literature and current research in western Kenya are tabulated and the relationships between critical water quality parameters described. The waters are chemically characterised with Na as the dominant cation and bicarbonate as the dominant anion and, while waters of obviously different sources are represented, the available chemical data point to a general classification of bicarbonate-Na-rich waters, even for the saline waters of Lake Magadi. Potassium and chloride are among the less abundandt constituents. The concentration level of nutrients (nitrate, phosphate and sulphate) is mostly lower than maxium permissible drinking water levels, and salinity is not yet a serious problem in water bodies that are exploited for domestic and industrial purposes. Fluoride levels are variable with the higher values occurring in waters in and around the Rift Valley. Limited analytical data for I in waters from the Eldoret, Kiambu and Nairobi areas indicate concentrations well above world average figures. Mean values of some key water quality indicators such as total dissolved solids, total suspended solids and heavy metals are well below the threshold for contaminated water. These values are however exceeded by several factors in saline waters of lakes and in some springs. Significant organic pollution is reflected by mean values of parameters such as biochemical oxygen demand and faecal coliforms. The present quality of most of the water bodies in this part of the country is considered to be adequate at present for domestic and other purposes, though a gradual decrease in quality is evident from the recent upsurge in industrial activities in the subregion.     

Developmental history of Paleozoic structure of Hsieh-fen-shan region of western Hunan and problem of boundary of Southwest Platform

The Paleozoic tectonic evolution of Hsieh-fen-shan, western Hunan, belongs to the Caledonian and Hercynian tectonic cycles. The base of the lower Paleozoic group is defined by a geographical unconformity; the Sinian system unconformably overlies the Pan-ch'i group of the Proterozoic. The Hsieh-fen movement represented by this unconformity had not transformed this area into a platform. The lower Paleozoic group may be divided into four structural stages: Lower Sinian, Upper Sinian-Cambrian, Lower Ordovician and Middle Ordovician-Silurian. These stages are represented by the corresponding sedimentary formations: lower terrigenous elastic formation, siliceous and carbonate rocks intercalated with shale, flush formation and upper terrigenous elastic. Due to the absence of magmatic activity it is believed to be of the miogeosyncline type. In the Variscan cycle this area was already consolidated and partly covered with upper Paleozoic neoplatform sediments. Hereditary characters of the early Paleozoic tectonic evolution are obvious. According to the isopachous maps there occurred a strong tectonic differentiation at the end of the Cambrian. In Early Ordovician a northeast extended flysch geosyncline formed, and to the west of it, an uplifted denudation region. The eastern part of the uplifted region rose strongly, in parallel with the flysch geosyncline and with similar dimensions, and may be considered a geanticline. The geosyncline and geanticline have asymmetrical forms, their neighboring flanks being steeper than their opposite ones. Consequently the existence of a so-called pre-Sinian Hsieh-fen axis has not been proved and the boundaries of the Southwestern Platform with the adjoining southeastern Caledonian fold may be located approximately along T'ao-yuan, Yuan-ling Chen-chi, Hsin-Hua and Ch'ien-yang. – Authors.     

Heavy metal contamination of a Greenland fjord system by mine wastes

Since 1973, about 500,000 tons/yr of metal-rich particulate tailings from a lead/zinc flotation mill have been discharged through a submarine outfall into a two fjord system on the west coast of Greenland. Differential solubilization of particulate metals by seawater, seasonal water mixing, and sill exchange tailings dispersal processes have resulted in high, but seasonally variable Zn, Cd, and Pb contamination of the water, and suspended particulate matter (SPM).Chemical partition of the SPM shows that most (85-99 percent) of the Pb, but relatively low proportions of Zn (14-26 percent) and Cd (10–20 percent) are weakly bound to the SPM. Such particulate metal characteristics allow the real time effects of tailings discharges and dispersal on the system to be traced even in the sediments where tailings accumulation is very slow (0.1 cm/yr).Fjord seaweeds and blue mussels also contain varying amounts of Zn, Pb, and Cd depending on the metal and their location relative to the tailings outfall and apparently responded almost instantly to the metal contamination, as did the water and SPM. High Pb concentrations in the fjord mussels most likely derive from the preferential uptake of available particulate Pb, whereas the seaweeds appear to derive most of their heavy metal concentrations from the dissolved phase. The evidence from this and other sites as well as from experimental work, indicates that any discharge of Pb-bearing particles into the marine environment either directly as mine wastes or indirectly as from natural runoff from current and former lead mining sites results in immediate lead contamination of the in situ mussel population.     

Geochemical constraints for the origin of thermal waters from western Turkey

The combined chemical composition, B and Sr isotopes, and the basic geologic setting of geothermal systems from the Menderes Massif in western Turkey have been investigated to evaluate the origin of the dissolved constituents and mechanisms of water–rock interaction. Four types of thermal water are present: (1) a Na–Cl of marine origin; (2) a Na–HCO₃ type with high CO₂ content that is associated with metamorphic rocks of the Menderes Massif; (3) a Na–SO₄ type that is also associated with metamorphic rocks of the Menderes Massif with H₂S addition; and (4) a Ca–Mg–HCO₃–SO₄ type that results from interactions with carbonate rocks at shallow depths. The Na–Cl waters are further subdivided based on Br/Cl ratios. Water from the Cumalı Seferihisar and Bodrum Karaada systems are deep circulated seawater (Br/Cl=sea water) whereas water from Çanakkale–Tuzla (Br/Cl<sea water) are from dissolution of Messinian evaporites. Good correlations between different dissolved salts and temperature indicate that the chemical composition of the thermal waters from non-marine geothermal systems is controlled by: (1) temperature dependent water–rock interactions; (2) intensification of reactions due to high dissolved CO₂ and possibly HCl gasses; and (3) mixing with overlying cold groundwater. All of the thermal water is enriched in B. The B isotopic composition (δ¹¹B=2.3‰ to 18.7‰; n=6) can indicate either leaching of B from the rocks, or B(OH)₃ degassing flux from deep sources. The large ranges in B concentrations in different rock types as well as in thermal waters from different systems suggest the water-rock mechanism. ⁸⁷Sr/⁸⁶Sr ratios of the thermal water are used to differentiate between solutes that have interacted with metamorphic rocks (⁸⁷Sr/⁸⁶Sr ratio as high as 0.719479) and carbonate rocks (low ⁸⁷Sr/⁸⁶Sr ratio of 0.707864).     

波生流垂向结构研究综述

波生流是海岸工程和海岸演变非常重要的动力因素之一,水深平均的平面二维波生流目前已有大量的研究成果,且广泛应用于工程实践中,其垂向变化往往对建筑物的稳定和海岸地貌的变化造成显著影响,例如堤前冲刷和岸滩蚀退.回顾了国内外对波生流垂向结构的研究与进展,从实验室与现场观测、理论分析以及数值模拟3个方面概述了该领域的研究现状与发展趋势,总结了已有研究存在的不足,提出需要进一步研究的课题.     

Average structure of the crust and upper mantle in East Africa

An average crust-mantle model has been derived for the East African Rift system based on a number of presently available seismic data. The inversion of experimentally determined spectral transfer ratios of long-period body waves recorded at stations AAE (Addis Ababa, Ethiopia), NAI (Nairobi, Kenya) and LWI (Lwiro, Zaïre) requires at least a two-layer crust. Except for station AAE. the observed P-wave delays can be accounted for by differences in the deduced crustal structure. Phase- and group-velocity measurements of Rayleigh waves along the path AAE-NAI provide additional information on the gross structure of the crust and upper mantle. Only a well-developed asthenosphere channel can explain the observed surface-wave dispersion. It is shown that the average model MS-71 permits a satisfactory interpretation of all the data presented in this paper.     

A revision of the Eurasian-African plate boundary in the western Mediterranean

The current picture of the plate configuration in the western Mediterranean suggests that Africa and Europe are separated by a transcurrent fault striking east-west through the Strait of Gibraltar and the Alboran Sea. Such a fault has been generally assumed because the Azores transform fault approaches the Horseshoe Seamounts north of Madeira and therefore could possibly extend through the Strait of Gibraltar. However, detailed geological studies in the Arc of Gibraltar and the Alboran Sea reveal no major east-west fault transecting the crust there. This finding is used here to revise the boundary between the African and Eurasian plates. It is suggested that the northern boundary of the African plate in the western Mediterranean is formed by the Hayes-Atlas Fault instead of the Azores transform.

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Zusammenfassung Nach gegenwärtiger Auffassung der Plattenkonfiguration im westlichen Mittelmeergebiet werden Afrika und Europa durch eine Ost-West verlaufende Transformverwerfung durch die Straße von Gibraltar und die Alboran See getrennt. Eine solche Verwerfung wird allgemein angenommen, da die Azoren-Transformverwerfung bis zum Horseshoe Seamount reicht und möglicherweise durch die Straße von Gibraltar fortgesetzt ist. Detailierte geologische Untersuchungen im Gibraltarbogen und der Alboran-See weisen jedoch nicht auf eine größere Ost-West Verwerfung hin, die dort die Kruste durchziehen könnte. Auf Grund dieses Befundes wird die Grenze zwischen der afrikanischen und der eurasischen Platte rediviert. Es wird vorgeschlagen, daß nicht die Azoren-Transformverwerfung, sondern die Hayes-Atlas-Verwerfung die nördliche Begrenzung der afrikanischen Platte im westlichen Mittelmeergebiet bildet.

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Résumé Le modèle habituellement admis des plaques dans la partie ouest de la Mediterranée suggère que l'Afrique et l'Europe sont séparées par une faille transcurrente est-ouest passant par le détroit de Gibraltar et la mer d'Alboran. Cette conception se base sur le fait que la faille transformante des Açores atteint le »Horseshoe Seamount« au nord de Madère et qu'elle pourrait se prolonger au travers du détroit de Gibraltar. Toutefois, des études géologiques détaillées dans l'Arc de Gibraltar et la mer d'Alboran ne révèlent pas de faille majeure (est-ouest) qui affecterait la croûte dans cette région. Cette observation amène une nouvelle interprétation du contact entre les plaques africaine et eurasiatique: il est suggéré que la bordure nord de la plaque africaine dans la partie ouest de la Méditerranée est formée par la faille de «Hayes-Atlas» et non par celle des Açores.

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, . , «» (Horseshoe Seamounts), , , . , - . . , , Hayes-Atlas.

     

The ordovician-silurian boundary on the western slope of the Subpolar Urals

Data obtained using different methods: paleontological, sedimentological, event stratigraphy and C-isotope chemostratigraphy of a unique succession of the Upper Ordovician and lower Silurian, located on the western slope of the Subpolar Urals, are presented in this work. The data obtained made it possible to revise some existing ideas about the texture of the Upper Ordovician succession and clarify the position of the Ordovician-Silurian boundary in the region. In addition, the Upper Ordovician Yaptiknyrd Formation was correlated with the synchronous formations in Scotland and Estonia.     

Preliminary 10Be chronology for the last deglaciation of the western margin of the Greenland Ice Sheet

The now acknowledged thinning of the Greenland Ice Sheet raises concerns about its potential contribution to future sea level rise. In order to appreciate the full extent of its contribution to sea level rise, reconstruction of the ice sheet's most recent last deglaciation could provide key information on the timing and the height of the ice sheet at a time of rapid climate readjustment. We measured ¹⁰Be concentrations in 12 samples collected along longitudinal and altitudinal transects from Sisimiut to within 10 km of the Isunguata Sermia Glacier ice margin on the western coast of Greenland. Along the longitudinal transect, we collected three perched boulders and two bedrocks. In addition, we sampled seven perched boulders along a vertical transect in a valley within 10 km of the Isunguata Sermia Glacier ice margin. Our pilot dataset constrains the height of the ice sheet during the Last Glacial Maximum (LGM) between 500 m and 840 m (including the 120 m relative sea level depression at the time of the LGM, 21 ka BP). From the transect we estimate the thinning of the ice sheet at the end of the deglaciation between 12.3 ± 1.5 ¹⁰Be ka (n = 2) and 8.3 ± 1.2 ¹⁰Be ka (n = 3) to be ～6 cm a^?1 over this time period. Direct dating of the retreat of the western margin of the Greenland Ice Sheet has the potential to better constrain the retreat rate of the ice margin, the thickness of the former ice sheet as well as its response to climate change. Copyright © 2008 John Wiley & Sons, Ltd.     

Coastal upwelling systems and Eastern boundary currents: A review of terminology

The use of terms such as Benguela Current or Peru Current System for different features of the oceanic circulation in the eastern parts of the oceans is reviewed. During 50 years of upwelling research these terms have been used for different oceanographic phenomena. It is argued that one reason for inconsistent use has been that names of currents were chosen on geographic rather than hydrodynamic principles, and that although knowledge of the dynamics of current systems expanded, it became increasingly difficult to modify the existing nomenclature accordingly. In order to provide a basis for a unified terminology, the dynamics of ocean currents in the eastern pans of the oceans are reviewed. It is argued that the coastal upwelling current system is a modification of the inshore part of the eastern boundary current and consequently should not be named as opposed to it but as part of it. It is suggested that the traditional geographic names be used for the eastern boundary currents and new names be assigned by international agreement to the coastal upwelling currents.     

Tracer transport in the Greenland Ice Sheet: constraints on ice cores and glacial history

The climate history and dynamics of the Greenland Ice Sheet are studied using a coupled model of the depositional provenance and transport of glacier ice, allowing simultaneous prediction of the detailed isotopic stratigraphy of ice cores at all the major Greenland sites. Adopting a novel method for reconstructing the age–depth relationship, we greatly improve the accuracy of semi-Lagrangian tracer tracking schemes and can readily incorporate an age-dependent ice rheology. The larger aim of our study is to impose new constraints on the glacial history of the Greenland Ice Sheet. Leading sources of uncertainty in the climate and dynamic history are encapsulated in a small number of parameters: the temperature and elevation isotopic sensitivities, the glacial–interglacial precipitation contrast and the effective viscosity of ice in the flow law. Comparing predicted and observed ice layering at ice core sites, we establish plausible ranges for the key model parameters, identify climate and dynamic histories that are mutually consistent and recover the past depositional elevation of ice cores to ease interpretation of their climatic records. With the coupled three-dimensional model of ice dynamics and provenance transport we propose a method to place all the ice core records on a common time scale and use discrepancies to adjust the reconstructed climate history. Analysis of simulated GRIP ice layering and borehole temperature profiles confirms that the GRIP record is sensitive to the dynamic as well as to the climatic history, but not enough to strongly limit speculation on the state of the Greenland Ice Sheet during the Eemian. In contrast, our study indicates that the Dye 3 and Camp Century ice cores are extremely sensitive to ice dynamics and greatly constrain Eemian ice sheet reconstructions. We suggest that the maximum Eemian sea-level contribution of the ice sheet was in the range of 3.5–4.5 m.     

On the spatial structure of currents across the Chesapeake and Delaware Canal

The Chesapeake and Delaware (C&;D) Canal is a man-made waterway connecting two of the largest estuaries on the east coast of the United States: Chesapeake Bay and Delaware Bay. A set of current meter data collected during April–May 1975 along two cross-sections of the C&;D Canal was used to examine the spatial distributions of the currents at tidal and subtidal time scales. The different responses of the Chesapeake and Delaware Bays to tidal and wind forcing produce significant differences in sea level fluctuations between the two ends of the canal. These alongcanal surface slopes produce significant barotropic current fluctuations at both tidal (semidiurnal and diurnal) and subtidal (2-d to 3-d) time scales. Under the influence of bottom friction, the barotropic currents near the surface are stronger than those at depth, but these currents do not exhibit significant lateral variations across the canal. On the other hand, the long-term flow in the canal exhibits strong lateral variability with eastward flow off the south shore of the canal and westward flow off the north shore of the canal. The lateral structure of the long-term flow may carry significant implications for the long-term exchange of material between the two bays.     

New evidence from beneath the western North Atlantic for the depth of glacial erosion in Greenland and North America

Interpretation of Deep Sea Drilling Project results and air-gun seismic profiles suggests that about 10⁶ km³ of sediment have been eroded from eastern North America and southern Greenland and deposited in the adjacent North Atlantic since the beginning of continental glaciation. This volume is a minimum estimate which does not account for sediment beneath the continental shelf nor that portion carried south of the Blake-Bahama Outer Ridge by the Western Boundary Undercurrent. It represents erosion of about 100 m of solid rock and indicates that more than 90% of the sediment eroded from these areas was deposited as sands, silts, and clays in the adjacent western North Atlantic. Glaciation accounts for between 55 and 95 m of this average 100 m, and fluvial processes account for the remainder. The documented erosion in part substantiates W. A. White's (1972, Geological Society of America Bulletin83, 1037–1056) hypothesis of deep erosion and exhumation of shield regions, but is not in agreement with the entire volume of erosion implied by his model.     

水电开发对河流水沙年内分配的影响分析

以中国西南重要的国际河流澜沧江为研究对象,借鉴"基尼系数"的构建思路,建立了水沙年内分配均匀度计算模型,并通过下游水沙与上游天然水沙和区域降水的关系,建立了河流水沙年内分配均匀度回归还原模型。还原结果与实测对比表明:漫湾、大朝山电站建设以来,澜沧江径流年内分配均匀度年均升高1.14%,变幅在-7.32%~10.81%之间,输沙年内分配均匀度年均降低0.23%,变幅在-18.98%~26.76%之间;漫湾、大朝山电站同时运行期间,澜沧江径流年内分配均匀度年均升高3.90%,输沙年内分配均匀度年均降低0.42%;大朝山水电站建设后,澜沧江径流年内分配均匀度升高而输沙年内分配变化不明显,说明大朝山水电建设后河流水沙关系发生了变化,但变化较小。该成果不仅能为相关研究提供方法借鉴,也为水电开发对澜沧江水沙年内分配影响程度的科学评估提供了新的依据。     

Interannual variability in the structure of the Kuroshio front along the western boundary of the North Pacific Ocean associated with the 1982 ENSO event

The vertically averaged temperature (T_av) over the upper 200 m of ocean in the W boundary of the N Pacific is used to detect changes in the strenght and path of the Kuroshio Front along the W boundary from Luzon (18° N) to Honshu (34° N) during the period, 1979–1982. During this time period the Kuroshio Front experienced significant interannual changes associated, both with the disappearance of the Kuroshio Meander in late 1980 and with the development of the 1982 ENSO event in early 1982. When the Kuroshio Meander S of Honshu disappeared in the fall of 1980, lasting until the summer of 1981, the intensity of the Kuroshio Front increased, associated with warmer than normal temperatures all along the W boundary of the N Pacific from Luzon to Honshu. The amplitude of the Kuroshio Meander was also correlated with fluctuations in the path of the Kuroshio Front at the Tokara Strait (30° N) and the Bashi Strait (20° N), and with the amplitude of the East China Sea Meander. The East China Sea Meander occurs W of the Ryukyu Islands at 25° N, formed when the Kuroshio Current enters the East China Sea from the Philippine Sea NE of Taiwan Island. It had large amplitude in winter and smaller amplitude in spring and early summer, similar to that in the Kuroshio Meander when it was present during this period. It also had related interannual variability; i.e., when the Kuroshio Meander disappeared in fall of 1980, the East China Sea Meander was weak. These results and earlier ones dealing with the Kuroshio Front E of Japan (e.g., White and He) indicate that fluctuations in the amplitude of the Kuroshio Meander S of Honshu were associated with similar changes in the meandering character over the entire Kuroshio Current System during this four year period. During the 1982 ENSO event, the temperature in the region of the Kuroshio Front in the W boundary became colder than normal, while the Kuroshio Meander S of Honshu and the East China Sea Meander NE of Taiwan Island developed larger amplitudes. This is consistent with the results of White and He, who found during this same time period that the mesoscale meander pattern in the Kuroshio Extension intensifying during the 1982 ENSO period. During this time, the magnitude of the Kuroshio Front all along the W boundary and in the Kuroshio Extension region was weaker in comparison with the three years prior to the 1982 ENSO event.     

川西前陆盆地中—新生代沉积迁移与构造转换

川西前陆盆地中—新生代各构造层的残余厚度展布和沉积特征分析发现,四川克拉通周缘的前陆盆地在晚三叠世时期发育于龙门山山前,明显属于龙门山褶皱逆冲构造载荷所形成的前渊凹陷;侏罗纪早期的沉积地层呈面状分布,没有表现出显著的挠曲沉降,指示了一个构造相对平静的阶段;中侏罗世早期前渊凹陷迁移至龙门山北段和米仓山山前,前渊沉积从晚三叠世的北东向转换为近东西向,广泛的湖泊相沉积预示了前陆盆地的欠充填状态;中侏罗世中晚期,川西盆地沉降中心又迁移到大巴山山前,相应的挠曲变形又从近东西向转化为北西向,构成了大巴山的前渊凹陷;晚侏罗世—早白垩世时期,沉降中心再次回到米仓山山前,巨厚的前渊凹陷沉积指示了米仓山冲断带的主要活动时期;白垩纪末—古近纪的前渊凹陷则跃迁至雅安—名山地区。川西前陆盆地的同造山沉降中心以四川盆地中心为核心在西部和北部呈弧形迁移,沉积序列不断更替和叠加。中生界各构造层底界构造图显示现今的构造低部位位于川西北地区和川西南地区,在川西北地区均有东西走向的等值线分布,而川西南地区等值线走向则为北东-南西向。因此分析认为,晚侏罗世至早白垩世的构造变形可能控制了川西盆地现今的地层变形,形成了川西北地区的南北向构造挤压结构,而晚期的新生代构造变形则主要体现在川西盆地的西南部,形成北东-南西向的地层展布特征。